Lecture 4: Membrane Permeability and Membrane Potential Flashcards
What is transmembrane potential (Vm)?
voltage difference across cell membrane
What is voltage signal?
difference in electrical potential energy between two points in electrical circuit
What is resting membrane potential (RMP)?
voltage across cell membrane at rest (negative inside relative to outside)
What is neural electrical signal?
transient change in Vm
What is receptor potential?
transmembrane potential difference produced by activation of a sensory receptor (non-electrical stimulus) – detection of touch, sound, light, chemical
What is synaptic potential?
potential difference across postsynaptic membrane produced by action of neurotransmitters at a neuronal synapse (the incoming signal that neuron receives) – excitatory or inhibitory
What is action potential?
rapid change (rise and fall) in membrane potential produced by impulse along membrane of muscle cell or nerve cell
What is depolarization?
Vm becomes less negative than RMP
What is hyperpolarization?
Vm becomes more negative than RMP
What is repolarization?
Vm returns toward RMP
What is Ohm’s Law?
V = IR
What is current (I)?
flow of charge from one location to another (can be positive current or negative current)
What is resistance (R)?
how difficult (energy-intensive) it is for current to flow
What is voltage (V)?
force that provides energy for current flow
What does Ohm’s Law predict?
mostly predicts what will happen to neuron’s Vm if you inject a particular current (I) into a neuron – BUT there are two exceptions
What are the two exceptions to what Ohm’s Law predicts?
exception 1: if sufficient positive current is injected, AP is triggered once a threshold level of membrane depolarization is reached (L6-7)
exception 2: even with subthreshold current injections, voltage doesn’t perfectly follow the timing of the current (L5)
What are the biophysical mechanisms that create electrical membrane potential signals?
ability of cell membranes to generate any transmembrane potential requires:
- unequal distributions of ion species across the two sides of a membrane (concentration differences)
- selective permeability of that membrane to different ion species
What does equilibrium mean?
forces pushing movement of ion one way or the other are balanced – ions are still moving across membrane, but Vm is no longer changing
What are the forces that establish equilibrium?
diffusive/osmotic pressure/force due to concentration differences
electromotive force due to increase in positive or negative charge on one side (and corresponding excess of negative or positive charge on original side)
Is diffusive/osmotic pressure/force or
electromotive force stronger?
EMF is very strong compared to diffusion, therefore net number of ions that must change sides to get to EK is tiny
Is equilibrium for a particular ion species always the same?
no – equilibrium for a particular ion species is always specific to the particular cell/system being studied
What are the factors that affect electromotive force (EMF)?
- transmembrane potential (Vm)
- ion valence (z)
- Faraday’s constant (F)
What are the factors that affect diffusive force?
- concentration gradient (out/in)
- temperature (T)
- gas (diffusion) constant (R)
Permeability is necessary to get a transmembrane potential. Why isn’t it part of the Nernst equation?
equilibrium potential represents the final stage – it doesn’t tell you anything about how quickly it will get there, only what the end result would be
it does not matter whether a membrane is very permeable to an ion – it will eventually get to its equilibrium potential
When does Vm depend on the relative permeability of the ions?
when more than one ion is permeable
In a system where multiple ions are permeable, how will each permeable ion change the membrane potential (Vm)?
each permeable ion will move the membrane potential (Vm) in the direction of its Eion proportionally to its (relative) permeability
In the GHK equation, what factors affect all ions equally?
- temperature (T)
- Faraday’s constant (F)
- gas constant (R)
In the GHK equation, what factors affect an individual ion’s Eion?
- log([ion]o[ion]i)
- valence/charge (z)
In the GHK equation, what factors affect an individual ion’s effect strength?
their relative permeability (Pion)
In the GHK equation, where did the z go?
all ions are monovalent
In the GHK equation, why are the inside and outside inverted for Cl- ?
to remove the -1 valence from the equation
What was the practical use of the GHK equation by neuroscientists?
to try to solve for P, not Vm
One Ion
What happens when there is selective permeability, and equal distribution?
charges on each side stay balanced
One Ion
What happens when there is selective permeability, and unequal distribution?
build up of charge on one side of membrane (Vm reaches equilibrium potential for that ion)
Two Ions
What happens when the permeabilities of two ions are equal (and concentration gradients are equal and opposite)?
no potential develops (Vm = 0 mV)
Two Ions
What happens when the permeabilities of two ions are unequal (and concentration gradients are equal and opposite)?
build up of charge on one side of membrane (Vm is closer to Eion of the more permeable ion)
What happens if relative permeabilities of two ions vary over time?
Vm will vary predictably over time in proportion to the relative permeabilities at any given time point
hypothesis 1: RMP reflects high relative permeability of whatever ion has closest Eion to RMP (ie. K+)
hypothesis 2: depolarization (ie. during AP) reflects a change toward higher permeability to Na
Although equal and opposite ion concentrations produce Vm of 0 mV, are the net ion fluxes balanced?
net ion fluxes are not actually balanced – different ion types are moving in each direction
What happens to concentration gradients over time?
concentration gradients run down/equalize
What is required in the long-term to maintain concentration gradients?
active transporter proteins
whenever there are ions with opposing equilibrium potentials in a cell, ion homeostasis requires mechanisms that can maintain concentration gradients in the face of slow run-down from ions travelling across the membrane
What are active transporter proteins?
pumps in cell membrane that maintain ion gradients over time
What does the Na/K-ATPase pump do?
maintains K+ and Na+ gradients (on a time scale of minutes to hours)
for every molecule of ATP hydrolyzed (one cycle), pump moves 3 Na+ out of cell, and brings 2 K+ in
Is the Na+/K+ ATPase pump an important determinant of RMP? Why?
NO – it is neither a determinant of any information carrying electrical signal in nervous system, because it does not move enough ions fast enough to matter (ie. within ms)
